1. Field of the Invention
The present invention relates to a Ti--Al alloy, having a light weight and a heat resistance property and being applicable to a rotary member such as a turbine wheel, a valve system member such as an engine valve or the like, and a process for producing the same.
2. Description of the Related Art
It has been known that 3 intermetallic compounds are present in a Ti--Al binary system alloy including titanium and aluminum, i.e., Ti.sub.3 Al, TiAl and TiAl.sub.3. Since the TiAl has a specific gravity of 3.8 and accordingly it is light, and since it has a high strength at an elevated temperature, it has been regarded as a promising one for a light-weighted and heat resistant material. However, since the TiAl lacks a ductility at room temperature, it is hard to process it plastically. In addition, when castings are formed with the Ti--Al binary system alloy by casting, shrinkage cavities are likely to occur in the castings. Accordingly, no favorable castings can be obtained.
Developments have been carried out so far in order to improve the properties of the Ti--Al binary system alloy. For instance, Japanese Unexamined Patent Publication No. 125634/1988 discloses a Ti--Al alloy comprising aluminum, boron and titanium, substantially the balance. Further, Japanese Unexamined Patent Publication No. 79335/1989 discloses a Ti--Al alloy comprising aluminum, at least one of nickel and silicon and titanium, substantially the balance. However, the Ti--Al alloys do not improve the properties of the alloy satisfactorily. Although the room temperature ductility is improved slightly when boron is added to Ti--Al alloy and the contents of carbon, oxygen and nitrogen are controlled, the castability of the Ti--Al alloy deteriorates. Thus, the addition of boron to Ti--Al alloy does not improve the castability satisfactorily.